An amphiphilic anionic azobenzene derivative, soluble in water, formed a monolayer on an aqueous subphase containing a water-soluble polycation. The monolayers were transferred onto solid substrate by the Langmuir-Blodgett (LB) technique. XPS measurements showed that ion exchange reaction proceeded almost completely at the air-water interface and that the ratio of the monomer unit of the polycation to the azobenzene was almost unity. UV/vis absorption and IR measurements indicated that the azobenzene photoisomerized reversibly in the LB films on alternate illumination with UV and vis light. Furthermore, a reversible morphological change induced by light was observed in the LB films with AFM. Before illumination, the surface of the single-layer LB film was very smooth with a surface undulation of less than 1 nm. On illumination with UV light, however, a number of hills, with the height of ca. 5 nm and the diameter of the base of ca. 100 nm, appeared on the film surface. These structures almost disappeared on illumination with vis light. This indicates that the widely accepted assumption that photoisomerization should not change the two-dimensional structures significantly does not hold in the present case.